The National Health Interview Survey (NHIS) is an important source of information on the health of the US population, including health status, health behaviors, acute and chronic conditions, healthcare access and utilization, and health insurance coverage. Fielded annually since 1957, it is the longest running national health survey in the US. Yet, the extraordinary potential of NHIS data for analysis of changing health status, behaviors, and healthcare use has rarely been exploited. A random sample of 100 empirical studies using NHIS data was reviewed revealing that 70% used a single year of data and fewer than 25% used data from before 1986. Comprehensive analyses with NHIS data are challenging with information scattered across years and data files; complexities of file structure, record linkage, and sampling weights; and changes in variables as well as sample designs over time. We report on the Integrated Health Interview Series (IHIS), a harmonized set of data and documentation based on the NHIS public use files from 1969 to the present. This integrated data series will facilitate cross-time comparisons of these invaluable health survey data using composite coding schemes to recode variables across time. The IHIS will enhance the value of NHIS data and promote new population health and health services research by allowing researchers to make consistent comparisons across four decades of dramatic change in public health, and thus to study the health status of Americans as a dynamic process. We describe the motivation for integrating the NHIS data, the components of a data integration project, and the methods used to create a four decade, cross-sectional time-series of population health data.